A lithium ion battery (LIB) has rapidly been growing in demand in recent years.
However, a problem remains in that each fine metal foreign matter is mixed into the battery in a production of the LIB. There has therefore been a demand for an increase in a detection accuracy of the metal foreign matter in the production process.
In general, as means to detect a foreign matter of micro size attached to a surface of a semiconductor wafer, a liquid crystal substrate or the like with high sensitivity, there has been used a method for detecting scattered light using a dark field configuration (DF configuration).
An electrode sheet being one of members of the LIB is one that an electrode mixture being a mixture containing an active material, a conductive additive and a binder has been coated on a metal foil being a collector. In an inspection of the foreign matter mixed in the film of the electrode mixture in its production process, there has been a demand for the detection of a metal foreign matter of small size (several micrometers or so) contained in the film with high sensitivity. Among battery materials, a positive electrode material is generally used as a component, such as LiCoO2 or the like being a few micrometers in a particle size and being several tens of micrometers at the same level as the thickness. Therefore, a transmittance of visible light becomes 0.1% or less. It is thus difficult to detect the foreign matter in the film by using the visible light.
Patent Document 1 discloses an inspection method of a paper sheet or the like, the method including the steps of irradiating the paper sheet or the like with terahertz light with a wavelength of 50 μm to 2 mm, detecting the terahertz reflected light reflected by front and back surfaces of the sheet or the like, and detecting an intensity of an interference due to a phase difference of the detected terahertz reflected light to detect a thickness of the sheet or the like.
Patent Document 2 discloses an apparatus of real-time terahertz tomography and terahertz spectral imaging, in which a technique for using a THz pulse and probe pulse light as a non-common axis and measuring a terahertz time waveform in a single shot and further irradiating an object to be measured with the THz pulse in a line condensing state and making unnecessary a scanning mechanism by using a two-dimensional imaging device as a detector with an objective of realizing a complete real-time measurement.
Patent Document 3 discloses a defect inspecting apparatus which changes the polarization axis of a polarizer according to the specific polarization axis of an object to be inspected and polarizing light from illuminating means to irradiate the object with the light and in which an analyzer aligns the polarization axis with the polarization axis of the polarizer and detects a change in penetration amount or a change in the reflection amount by optical detecting means through the transmitted light from the object or the reflected light, thereby detecting defects of the object.
Patent Document 4 discloses a surface inspecting apparatus which uses a microwave having a wavelength of 10 μm to 1 mm to detect a reflected wave from a target surface to be measured, thereby determining the surface shape of a metal.
Patent Document 5 discloses an inspection method which allows the infrared sufficiently longer in wavelength than the diameter of powder particles to pass through the inside of the power particles to thereby examine an internal structure of a subject constructed by aggregating the powder particles.
Patent Document 6 discloses an apparatus which irradiates an object to be inspected with a pulse-shaped or continuous sub terahertz electromagnetic wave having a wavelength of 600 μm to 3 mm (0.5 THz to 100 GHz), and performs an inspection of a foreign matter in powder particles by utilizing a difference in a propagation time or a difference in a transmittance due to its substance.
In Non-Patent Document 1, wavelength dependences of refractive indices and absorption coefficients of various metals, composite oxide or the like are described.